1. Field of the Invention
The present invention relates to a method of and device for inspecting a pattern on a printed board, and more particularly relates to an inspecting method for judging a relative positional discrepancy between a wiring pattern and a through hole (in which a mini-via hole is included).
2. Description of the Prior Art
With advancement in miniaturization and realization of high performance for electronic parts, patterns on printed board circuits has been formed in microstructure as well as in high density. This trend has brought about requirements of narrowing pattern width and decreasing the diameter of through holes. In particular, the diameter of through holes which are employed as conductive holes in multi-layer substrates have been decreased. The diameter of this type of through holes was conventionally 0.8 mm. At present, the diameter has been further decreased, and through holes having diameter of 0.1 to 0.5 mm, known as mini-via holes, are employed.
As the diameter of through holes has been decreased, there have arisen demands for new techniques in various fields related to treating through holes including plating, drilling, processing and reliability testing.
In general, accuracy in drilling processes is inferior to that in photoetching processes, so that in many cases drilled through holes deviate from desired patterns. However, a slight positional discrepancy has only a slight influence on electronic reliability of substrates, since sufficiently large lands are provided around the through holes which have a diameter of about 0.8 mm.
With advancement in the technique of decreasing diameter of through holes, smaller lands are formed around the holes. Accordingly, it has become difficult to guarantee absolute accuracy in providing a hole which is to be employed as a through hole within a land, and there has arisen the problem of electronic reliability of printed circuit boards. Specifically, electronic reliability has deteriorated due to the positional discrepancy of the hole. Therefore, inspection on positional discrepancy of through holes becomes extremely important.
The inspection on positional discrepancy of holes requires two-way approaches; electronic inspection and visual inspection. For visual inspection, an inspecting device for detecting light which leaks from cracks of a metal plate is well known. However, for multi-layered board construction, there are additional problems to be solved. Furthermore, this inspecting device cannot be applied to inspecting pattern breaks which are caused by relative positional discrepancy between through holes and wiring patterns.
FIGS. 18A and 18B illustrate the positional relations between a land R and a through hole H within a wiring pattern P. In FIG. 18A, the center O of the through hole H coincides with the center of the land R, and the pattern is good. In FIG. 18B, the center O of the through hole H is deviated from the center of the land R, and part of the through hole H projects into the exterior of the land R. The size of this projecting part can be expressed by the aperture angle .theta.. When the aperture angle .theta. is larger than a predetermined reference value, it is judged that the pattern break is defective.
Whether pattern breaks are good or defective can be determined by detecting the aperture angle .theta.. Most conventional inspecting techniques rely upon human eyes using a magnifying lens or the like to detect the aperture angle .theta..
A technique for automatically detecting this aperture angle .theta. is disclosed by Japanese Patent Application No. 1-82117, which was filed by the present applicant. That technique referring is explained herein with reference to FIG. 19. According to that technique, the aperture angle .theta. is obtained by detecting an overlapped region WR between a pattern RI of the land R which is expanded at a proper magnification and a contour RP of the hole H which is also expanded and magnified at a proper magnification.
This technique is excellent, as compared to difficulties of binary techniques, when applied in treating an opening edge of the through hole. However, since this technique relies upon the aperture angle .theta. to judge whether pattern breaks are good or defective, there is a high probability of misjudgement in this technique in the case shown in FIG. 20A where a plurality of patterns LI of lines L enter a pattern RI of the land. In the case shown in FIG. 20B where the land pattern RI is somewhat deformed, or when there is significant influence of quantization errors or noise which cannot be cut through a noise filter. It is not possible to obtain an accurate aperture angle which is represented by dashed lines in FIGS. 20A and 20B, respectively.